1. Technical Field
This invention relates generally to electronic devices, and more particularly, to an approach for connecting electronic devices on opposite sides of a circuit board.
2. Background Art
FIG. 1 illustrates an electronic device in the form of a packaged semiconductor device 30. The device 30 includes a ball grid array 32 on one side thereof (shown on the side of the device 30 facing the viewer of FIG. 1), with the number of balls of the array shown greatly reduced for clarity. The ball grid array includes a plurality of conductive balls which make up contacts formed in rows, patterns or other arrangements. In this embodiment, rows are depicted For example, the row 36 of contacts 34 includes contacts 34-4, 34-3, 34-2, 34-1 (reading from left to right in FIG. 1). The contacts 34 communicate with a semiconductor chip (not shown) inside the package, as is well known.
FIG. 2 illustrates the device 30 of FIG. 1 rotated 180 degrees about the axis X shown in FIG. 1. As such, the contacts 34 are now on the side of the device 30 facing away from the viewer of FIG. 2, and the order of the contacts in row 36 (again reading from left to right in FIG. 2) is reversed relative to FIG. 1.
With reference to FIG. 3, many of today's electronic systems utilize double-sided circuit boards with electronic devices mounted on both sides of the board, with the contacts thereof connected to traces on the circuit board. Each contact of one device is connected to a chosen contact of the device on the other side by means of a conductive connection passing through the circuit board. Signals which may be transferred from one device to another through the circuit board may include input/output signals (including, but not limited to, address signals, data signals, power and ground connections, clock signals, etc). In the present example, it may be desirable to connect identical devices 30A, 30B (each identical to the device 30 of FIGS. 1 and 2) through the circuit board 38, with corresponding contacts of the devices 30A, 30B connected. That is, for example, contact 34A-1 of device 30A is to be connected to contact 34B-1 of device 30B, contact 34A-2 of device 30A is to be connected to contact 34B-2 of device 30B, contact 34A-3 of device 30A is to be connected to contact 34B-3 of device 30B, and contact 34A-4 of device 30A is to be connected to contact 34B-4 of device 30B. These connections 40, 42, 44, 46 are shown in schematic form in FIG. 3. FIG. 4 illustrates the actual complexity of making these connections. The layers 38-1, 38-2, 38-3, 38-4, 38-5, 38-6 of the circuit board 38, as shown in FIG. 4, must have formed therein complex passages from one side to the other for provision of the conductive connectors 40, 42, 44, 46 therethrough so as to make appropriate connection as specified above. Individual portions of the connectors must be formed layer-by-layer and must of course be very accurately placed. This, along with the need for multiple layers for the circuit board 38, greatly increases the complexity and cost of the overall electronic structure.
One way of mirroring and matching front-side contacts with back-side contacts is to mirror the component die pad orientations on the semiconductor chip. However, present chip mask costs, even for only a partial mask set change, make this option prohibitively costly.
Another possible solution involves mirroring the package contact positioning. However, simply mirroring the package contact positioning with no modifications to the chip bond pad layout can detrimentally affect package signal integrity, and thus product performance.
Therefore, what is needed is an approach which overcomes these problems by greatly simplifying the connections as described above through the circuit board.